This invention relates to pressure transducers having a piezoelectric element for pressure measuring at high temperatures, particularly for the combustion chamber pressure of internal combustion engines, in which the combustion chamber pressure is measured by deformation of the piezoelectric element or elements.
The pressure measurement in internal combustion engines has been, since the early stages of engine development, one of the most important aids in the evaluation of combustion. The pressure is a state variable of the thermodynamic processes in the engine. Thus, the measuring of pressure has increasing importance in the operational monitoring of large diesel engines. Typically, a pressure measuring device of robust construction was required so that, even without forced cooling, long lay-up times are attained. In addition, such pressure measuring devices should be high - temperature independent with respect to the signal transmission factor, insensitive to temperature shock and protected against excessively high temperatures.
Force-cooled pressure measuring devices for thermodynamic investigations have long been used for pressure measurement in internal combustion engines. The piezoelectric measuring technique, employing a pressure transducer having a quartz-monocrystal, now enjoys widespread use. However, such transducer are subjected to high mechanical and thermal and especially shock stresses during combustion in the combustion chamber. Most attempts at improvements have thus been directed to compensating for such stress, to the pressure responsive membrane, its attachments, the piezoelectric measuring element and the materials employed.
The limited life span of cooled pressure measurement devices, the difficulty in handling and other factors are decisive in the development of uncooled pressure measuring devices, including those having piezoelectric elements. Cooled pressure measuring devices, having longitudinal or transverse mode of loading the quartz are described, for example, by Dubendorfer, U. and Wolfer, P. in a publication entitled "State-of-the-Art Pressure Measurments in Combustion Engines" in ISA Transactions, Volume 24, Number 2, 1985 Pages 35 to 38.